diff --git a/src/models/chat_session.ts b/src/models/chat_session.ts
index 431f9ee4..34e8b55d 100644
--- a/src/models/chat_session.ts
+++ b/src/models/chat_session.ts
@@ -33,6 +33,7 @@ import {
   SafetySetting,
   StartChatSessionRequest,
   StreamGenerateContentResult,
+  ThinkingConfig,
   Tool,
 } from '../types/content';
 import {ToolConfig} from '../types';
@@ -59,6 +60,7 @@ export class ChatSession {
   private readonly safetySettings?: SafetySetting[];
   private readonly tools?: Tool[];
   private readonly toolConfig?: ToolConfig;
+  private readonly thinkingConfig?: ThinkingConfig;
   private readonly apiEndpoint?: string;
   private readonly systemInstruction?: Content;
 
@@ -83,6 +85,7 @@ export class ChatSession {
     this.safetySettings = request.safetySettings;
     this.tools = request.tools;
     this.toolConfig = request.toolConfig;
+    this.thinkingConfig = request.thinkingConfig;
     this.apiEndpoint = request.apiEndpoint;
     this.requestOptions = requestOptions ?? {};
     if (request.systemInstruction) {
@@ -135,6 +138,7 @@ export class ChatSession {
       tools: this.tools,
       toolConfig: this.toolConfig,
       systemInstruction: this.systemInstruction,
+      thinkingConfig: this.thinkingConfig,
     };
 
     const generateContentResult: GenerateContentResult = await generateContent(
@@ -220,6 +224,7 @@ export class ChatSession {
       tools: this.tools,
       toolConfig: this.toolConfig,
       systemInstruction: this.systemInstruction,
+      thinkingConfig: this.thinkingConfig,
     };
 
     const streamGenerateContentResultPromise = generateContentStream(
@@ -269,6 +274,7 @@ export class ChatSessionPreview {
   private readonly safetySettings?: SafetySetting[];
   private readonly tools?: Tool[];
   private readonly toolConfig?: ToolConfig;
+  private readonly thinkingConfig?: ThinkingConfig;
   private readonly apiEndpoint?: string;
   private readonly systemInstruction?: Content;
   private readonly cachedContent?: string;
@@ -294,6 +300,7 @@ export class ChatSessionPreview {
     this.safetySettings = request.safetySettings;
     this.tools = request.tools;
     this.toolConfig = request.toolConfig;
+    this.thinkingConfig = request.thinkingConfig;
     this.apiEndpoint = request.apiEndpoint;
     this.requestOptions = requestOptions ?? {};
     this.cachedContent = request.cachedContent;
@@ -347,6 +354,7 @@ export class ChatSessionPreview {
       toolConfig: this.toolConfig,
       systemInstruction: this.systemInstruction,
       cachedContent: this.cachedContent,
+      thinkingConfig: this.thinkingConfig,
     };
 
     const generateContentResult: GenerateContentResult = await generateContent(
@@ -434,6 +442,7 @@ export class ChatSessionPreview {
       toolConfig: this.toolConfig,
       systemInstruction: this.systemInstruction,
       cachedContent: this.cachedContent,
+      thinkingConfig: this.thinkingConfig,
     };
 
     const streamGenerateContentResultPromise = generateContentStream(
diff --git a/src/models/generative_models.ts b/src/models/generative_models.ts
index 1e2820e9..213ca5a4 100644
--- a/src/models/generative_models.ts
+++ b/src/models/generative_models.ts
@@ -129,17 +129,16 @@ export class GenerativeModel {
   async generateContent(
     request: GenerateContentRequest | string
   ): Promise<GenerateContentResult> {
-    request = formulateRequestToGenerateContentRequest(request);
-    const formulatedRequest =
-      formulateSystemInstructionIntoGenerateContentRequest(
-        request,
-        this.systemInstruction
-      );
+    const formulatedRequest = formulateRequestToGenerateContentRequest(request);
+    const finalRequest = formulateSystemInstructionIntoGenerateContentRequest(
+      formulatedRequest,
+      this.systemInstruction
+    );
     return generateContent(
       this.location,
       this.resourcePath,
       this.fetchToken(),
-      formulatedRequest,
+      finalRequest,
       this.apiEndpoint,
       this.generationConfig,
       this.safetySettings,
@@ -176,17 +175,16 @@ export class GenerativeModel {
   async generateContentStream(
     request: GenerateContentRequest | string
   ): Promise<StreamGenerateContentResult> {
-    request = formulateRequestToGenerateContentRequest(request);
-    const formulatedRequest =
-      formulateSystemInstructionIntoGenerateContentRequest(
-        request,
-        this.systemInstruction
-      );
+    const formulatedRequest = formulateRequestToGenerateContentRequest(request);
+    const finalRequest = formulateSystemInstructionIntoGenerateContentRequest(
+      formulatedRequest,
+      this.systemInstruction
+    );
     return generateContentStream(
       this.location,
       this.resourcePath,
       this.fetchToken(),
-      formulatedRequest,
+      finalRequest,
       this.apiEndpoint,
       this.generationConfig,
       this.safetySettings,
@@ -361,10 +359,10 @@ export class GenerativeModelPreview {
   async generateContent(
     request: GenerateContentRequest | string
   ): Promise<GenerateContentResult> {
-    request = formulateRequestToGenerateContentRequest(request);
-    const formulatedRequest = {
+    const formulatedRequest = formulateRequestToGenerateContentRequest(request);
+    const finalRequest = {
       ...formulateSystemInstructionIntoGenerateContentRequest(
-        request,
+        formulatedRequest,
         this.systemInstruction
       ),
       cachedContent: this.cachedContent?.name,
@@ -373,7 +371,7 @@ export class GenerativeModelPreview {
       this.location,
       this.resourcePath,
       this.fetchToken(),
-      formulatedRequest,
+      finalRequest,
       this.apiEndpoint,
       this.generationConfig,
       this.safetySettings,
@@ -410,10 +408,10 @@ export class GenerativeModelPreview {
   async generateContentStream(
     request: GenerateContentRequest | string
   ): Promise<StreamGenerateContentResult> {
-    request = formulateRequestToGenerateContentRequest(request);
-    const formulatedRequest = {
+    const formulatedRequest = formulateRequestToGenerateContentRequest(request);
+    const finalRequest = {
       ...formulateSystemInstructionIntoGenerateContentRequest(
-        request,
+        formulatedRequest,
         this.systemInstruction
       ),
       cachedContent: this.cachedContent?.name,
@@ -422,7 +420,7 @@ export class GenerativeModelPreview {
       this.location,
       this.resourcePath,
       this.fetchToken(),
-      formulatedRequest,
+      finalRequest,
       this.apiEndpoint,
       this.generationConfig,
       this.safetySettings,
diff --git a/src/types/content.ts b/src/types/content.ts
index a227e47f..1d79f383 100644
--- a/src/types/content.ts
+++ b/src/types/content.ts
@@ -99,6 +99,17 @@ export declare interface CountTokensResponse {
   totalBillableCharacters?: number;
 }
 
+/**
+ * The configuration for thinking features.
+ * @public
+ */
+export declare interface ThinkingConfig {
+  /** Optional. Indicates whether to include thoughts in the response. If true, thoughts are returned only when available. */
+  includeThoughts?: boolean;
+  /** Optional. Indicates the thinking budget in tokens. This is only applied when enable_thinking is true. */
+  thinkingBudget?: number;
+}
+
 /**
  * Params used to call the getGenerativeModel method.
  */
@@ -233,6 +244,8 @@ export declare interface GenerationConfig {
    * If set, a compatible responseMimeType must also be set.
    */
   responseSchema?: ResponseSchema;
+  /** Optional. The configuration for thinking features. */
+  thinkingConfig?: ThinkingConfig;
 }
 
 /**